Step-edged grain boundary Josephson junction with 5 to 30 degrees inclined angle

ABSTRACT

A Josephson junction device comprising a single crystalline substrate including a principal surface having two horizontal planes and a smooth slope between the two horizontal planes, and an oxide superconductor thin film formed on the principal surface of the substrate. The oxide superconductor thin film includes a first and a second superconducting portions of a single crystalline oxide superconductor respectively positioned on the two horizontal planes of the substrate, a junction portion of a single crystalline oxide superconductor having a different crystal orientation from the two superconducting portions positioned on the slope of the substrate and two grain boundaries between each of the two superconducting portions and the junction portion. The grain boundaries constitutes one weak link of the Josephson junction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Josephson junction device formed ofoxide superconductor, and more specifically to a Josephson junctiondevice of an oxide superconductor, of which the weak link of theJosephson junction is constituted of grain boundaries of oxidesuperconductor.

2. Description of Related Art

A Josephson junction device which is one of superconducting devices canbe realized in various structures. Among the various structures, themost preferable structure in practice is a stacked junction realized bya thin non-superconductor layer sandwiched between a pair ofsuperconductors. However, a point contact type junction, a Dayem bridgetype junction and a variable thickness bridge type junction which arecomposed of a pair of superconductor regions which are weakly linked toeach other also exhibit Josephson effect. In general, these Josephsonjunctions have fine structures in which the superconductor and/ornon-superconductor are composed of thin films.

In order to realize a stacked type junction by using an oxidesuperconductor, a first oxide superconductor thin film, anon-superconductor thin film and a second oxide superconductor thin filmare stacked on a substrate in the named order.

In the above mentioned stacked type junction, an insulator MgO etc., asemiconductor Si etc., and a metal Au etc. are used for thenon-superconductor layers so that each superconducting junction hasdifferent properties for each applications.

The thickness of the non-superconductor layer of the stacked typejunction is determined by the coherence length of the superconductor. Ingeneral, the thickness of the non-superconductor layer of the stackedtype junction must be within a few times of the coherence length of thesuperconductor. Since oxide superconductor materials have a very shortcoherence length, therefore, a thickness of a non-superconductor layermust be about a few nanometers.

However, the superconductor layers and the non-superconductor layer ofthe stacked type junction must be of high crystallinity for favorablejunction properties, which are composed of single crystals or composedof polycrystals which are orientated in almost same direction. It isdifficult to stack an extremely thin and high crystallinenon-superconductor layer on an oxide superconductor layer. Additionally,it is very difficult ,to stack a high crystalline oxide superconductorlayer on the non-superconductor layer stacked on an oxide superconductorlayer. Though the stacked structure including a first oxidesuperconductor layer, a non-superconductor layer and a second oxidesuperconductor layer is realized, the interfaces between the oxidesuperconductor layers and the non-superconductor layer are not in goodcondition so that the stacked type junction does not function in goodorder.

In order to manufacture a point contact type junction, a Dayem bridgetype junction and a variable thickness bridge type junction by usingoxide superconductor, very fine processings which realize a weak link ofa pair of superconductor are necessary. It is very difficult to conducta fine processing with good repeatability.

The point contact type junction has been formed of two oxidesuperconductor thin films which are in contact with each other in aextremely small area which constitutes the weak link of the Josephsonjunction.

The Dayem bridge type junction has been formed of a constant thicknessoxide superconductor thin film which is formed on a substrate and whichis patterned in a plan view, so that a superconductor thin film regionhaving a greatly narrow width is formed between a pair of superconductorthin film regions having a sufficient width. In other words, the pair ofsuperconductor thin film regions having a sufficient width are coupledto each other by the superconductor thin film region having the greatlynarrow width. Namely, a weak link of the Josephson junction in thesuperconductor thin film is formed at the greatly narrow width region.

On the other hand, the variable thickness bridge type junction has beenformed of an oxide superconductor thin film of a sufficient thicknesswhich is formed on a substrate and which is partially etched or thinnedin a thickness direction, so that a thinned oxide superconductor thinfilm portion is formed between a pair of superconductor thin filmportions having the sufficient thickness. In other words, the pair ofsuperconductor thin film portions having the sufficient thickness arecoupled to each other by the thinned oxide superconductor thin filmportion. Accordingly, a weak link of the Josephson junction is formed atthe reduced thickness portion of the oxide superconductor thin film.

As would be understood from the above, a characteristics of theJosephson device has a close relation to the contact area of thesuperconductor thin film in the point contact type Josephson device, thewidth of the superconductor thin film region having the extremely narrowwidth in the Dayem bridge type Josephson device, and to the thickness ofthe thinned oxide superconductor thin film portion in the variablethickness bridge type Josephson device, both of which form the weak linkof the Josephson junction. Therefore, in order to obtain a desiredcharacteristics with a good repeatability, a high precision on asub-micron level of the processing such as the etching is required.

The Dayem bridge type Josephson device can be said to be more preferablethan the variable thickness bridge type Josephson device, since theDayem bridge type Josephson device has a relatively planer surface,which is preferred in a integrated circuit. However, in order to formthe weak link in the Dayem bridge type Josephson device, it is requiredto pattern an oxide superconductor thin film having the thickness on theorder of 0.5 μm to 1.0 μm into a width of not greater than 0.2 μm.However, it is very difficult to conduct this fine patterning with goodrepeatability.

On the other hand, in the variable thickness bridge type Josephsondevice, the very fine pattering is not required in order to form theweak link. However, it is very difficult to uniformly control theremaining thickness of the thinned portion forming the weak link. Inaddition, the variable thickness bridge type Josephson device cannothave a planer surface by nature. This is not preferable to theintegrated circuit application.

Therefore, in the prior art, it is almost impossible to manufacture asuperconducting device for example a dc SQUID (superconducting quantuminterference device) which has plural homogeneous Josephson junctionsutilizing an oxide superconductor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aJosephson junction device composed of an oxide superconductor materialincluding a weak link of the Josephson junction and an almost planarsurface, which have overcome the above mentioned defects of theconventional ones.

Another object of the present invention is to provide a Josephsonjunction device composed of an oxide superconductor material includingplural weak links of the Josephson junction which have homogeneouscharacteristics.

The above and other objects of the present invention are achieved inaccordance with the present invention by a Josephson junction devicecomprising a single crystalline substrate including a principal surfacehaving two horizontal planes and a smooth slope between the twohorizontal planes, and an oxide superconductor thin film formed on theprincipal surface of the substrate, which includes a first and a secondsuperconducting portions of a single crystalline oxide superconductorrespectively positioned on the two horizontal planes of the substrate, ajunction portion of a single crystalline oxide superconductor having adifferent crystal orientation from the two superconducting portionspositioned on the slope of the substrate and two grain boundariesbetween each of the two superconducting portions and the junctionportion, which constitutes one weak link of the Josephson junction.

In a prior art, there is known a Josephson junction device utilizing anoxide superconductor which includes a substrate having a step on asurface and an oxide superconductor thin film formed on the steppedsurface of the substrate. This conventional Josephson junction devicehas a step on its upper surface. The step becomes one of obstacles forforming conductor wirings or a multi-layered structure.

The Josephson junction device in accordance with present invention has asubstantially planar upper surface. Therefore, it become easy to formconductor wirings or to realize a multi-layered structure. This planarupper surface can be realized by the smooth slope of the substrate.

Preferably, the oxide superconductor thin film includes two junctionportions, each of which includes the grain boundaries which constitutethe weak link of the Josephson junction. In this case, it is preferablethat the oxide superconductor thin film is formed in an rectangular ringshape which includes two junction portions each of which includes thegrain boundaries which constitute the weak link of Josephson junction,so that the Josephson junction device constitutes a dc SQUID.

In a preferred embodiment, the slope inclines at an angle of 5° to 30°.If the gradient is smaller than 5°, the crystal orientation of the oxidesuperconductor of the junction portion does not change and no grainboundary is formed. If the gradient is larger than 30°, a step is formedon the upper surface of the oxide superconductor thin film so that itdoes not become planar. In addition, if the gradient is too large, thegrain boundaries become unstable, so that the value of the criticalcurrent of the Josephson junction becomes unstable.

In the Josephson junction device in accordance with the presentinvention, the length of the slope of the substrate is determined by thedifference of the heights of the horizontal planes of the principalsurface of the substrate. The difference of the heights of thehorizontal planes is preferably 100 to 500 nanometers. If the differenceis smaller than 100 nanometers, the oxide superconductor of the junctionportion does not have enough crystallinity so that the characteristicsof the Josephson junction is spoiled. On the other hand, if thedifference is larger than 500 nanometers, the grain boundaries becometoo far away from each other so that each of the weak linksindependently operates.

The preferable difference of the heights of the horizontal planes isrelative to the thickness of the oxide superconductor thin film. In theJosephson junction device in accordance with the present invention, itis preferable that the oxide superconductor thin film has a thickness of100 nanometers to 1 μm.

It is preferable that the two superconducting portions are formed ofc-axis orientated oxide superconductor thin films. Since, the oxidesuperconductor has the largest critical current density in a directionperpendicular to the c-axis of the crystal. Therefore, if thesuperconducting portions are formed of c-axis orientated oxidesuperconductor thin films, larger current can flow through the Josephsonjunction device.

In the Josephson junction device in accordance with the presentinvention, the substrate is limited to an MgO substrate, since thecrystal orientation of an oxide superconductor thin film deposited onthe MgO substrate is sensitively affected by the condition of thedeposition surface of the MgO substrate. If a portion of the depositionsurface inclines, the crystal orientation of the oxide superconductorthin film is deviated on the portion corresponding to the gradient ofthe portion. The Josephson junction device in accordance with thepresent invention utilizes this property.

In one preferred embodiment, the junction portion of the oxidesuperconductor thin film has a narrower width than other portions. Thisnarrow junction portion stables the Josephson effect generated at thejunction portion. The narrow junction portion of the oxidesuperconductor thin film preferably has a width of 1.5 to 15 μm and alength of 5 to 50 μm.

In a preferred embodiment, the oxide superconductor is formed ofhigh-T_(c) (high critical temperature) oxide superconductor,particularly, formed of a high-T_(c) copper-oxide type compound oxidesuperconductor for example a Y-Ba-Cu-O compound oxide superconductormaterial, a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and aTl-Ba-Ca-Cu-O compound oxide superconductor material.

The above and oilier objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sectional view of an embodiment of theJosephson junction device in accordance with the present invention;

FIGS. 2 is a diagrammatic plane view of the Josephson junction deviceshown in FIG. 1; and

FIGS. 3 is a diagrammatic plane view of another embodiment of theJosephson junction device in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Referring to FIG. 1, them is shown a diagrammatic sectional view of anembodiment of the Josephson junction device in accordance with thepresent invention. The Josephson junction device includes an MgO (100)single crystalline substrate 3 and a Y₁ Ba₂ Cu₃ O_(7-x) oxidesuperconductor thin film 1 formed on an upper surface of the substrate 3having a thickness of 300 nanometers. The upper surface of the MgO (100)single crystalline substrate 3 has two horizontal planes 31 and 32((100) plane) having different heights, and a smooth slope 33 betweenthe horizontal planes 31 and 32. The slope 33 inclines at an angle of20° and the difference of the heights of the horizontal planes 31 and 32is 200 nanometers. However, the upper surface of the Y₁ Ba₂ Cu₃ O_(7-x)oxide superconductor thin film 1 has a small swelling so that it can betaken as a plane.

The Y₁ Ba₂ Cu₃ O_(7-x) oxide superconductor thin film 1 has twosuperconducting portions 11 and 12, and a junction portion 13. Thesuperconducting portions 11 and 12 are positioned on the horizontalplanes 31 and 32 of the substrate 3, and the junction portion 13 ispositioned on the slope 33 of the substrate 3. The superconductingportions 11 and 12 are constituted of c-axis orientated Y₁ Ba₂ Cu₃O_(7-x) oxide superconductor thin films which have c-axes of crystalsperpendicular to the horizontal planes 31 and 32. On the other hand,c-axes of Y₁ Ba₂ Cu₃ O_(7-x) oxide superconductor crystal of thejunction portion 13 is perpendicular to the slope 33, so that thecrystal orientation of the oxide superconductor of the junction portion13 is different from those of the superconducting portions 11 and 12.Therefore, grain boundaries 41 and 42 are formed between thesuperconducting portion 11 and the junction portion 13, and between thesuperconducting portion 12 and the junction portion 13. Each of thesegrain boundaries 41 and 42 constitutes a weak link of the Josephsonjunction. However, the grain boundaries 41 and 42 are formed in closeproximity to each other so that the weak links of the Josephson junctionoperate as one weak link of the Josephson junction.

FIG. 2 shows a plan view of the Josephson junction device shown inFIG. 1. As shown in FIG. 2, the superconducting portion 11 and 12 havethe same pentagonal shape arranged symmetrically so as to formsuperconducting electrodes, and they are connected by the narrowjunction portion 13. Only the junction portion 13 is positioned on theslope 33 of the substrate 3.

In the above Josephson junction device, the gradient of the slope 33 ofthe substrate 3 is preferably 5° to 30°. If the gradient is smaller than5°, the crystal orientation of the oxide superconductor of the junctionportion 13 does not change and no grain boundary is formed. If thegradient is larger than 30°, a step is formed on the upper surface ofthe oxide superconductor thin film 1 so that it does not become planar.In addition, if the gradient is too large, the grain boundaries 41 and42 become unstable, so that the value of the critical current of theJosephson junction becomes unstable.

The difference of the heights of the horizontal planes 31 and 32 ispreferably 100 to 500 nanometers. If the difference is smaller than 100nanometers, the oxide superconductor of the junction portion 13 does nothave enough crystallinity so that the characteristics of the Josephsonjunction is spoiled. On the other hand, if the difference is larger than500 nanometers, the grain boundaries 41 and 42 become too far away fromeach other so that each of the weak links independently operates.

The preferable difference of the heights of the horizontal planes 31 and32 is relative to the thickness of the oxide superconductor thin film 1.It is preferable that the oxide superconductor thin film 1 has athickness of 100 nanometers to 1 μm.

The substrate 3 is limited to an MgO substrate, since the crystalorientation of an oxide superconductor thin film deposited on the MgOsubstrate is sensitively affected by the condition of the depositionsurface of the MgO substrate. If a portion of the deposition surfaceinclines, the crystal orientation of the oxide superconductor thin filmis deviated on the portion corresponding to the gradient of the portion.The Josephson junction device in accordance with the present inventionutilizes this property.

The above Josephson junction device was manufactured by the followingprocess.

At first, an MgO (100) substrate was etched by an ion-milling using Arions so as to form the slope 33 inclining at an angle of 20° on theprinciple (100) plane. A reactive ion etching could be used for thisprocessing.

Thereafter, a Y₁ Ba₂ Cu₃ O_(7-x) oxide superconductor thin film 1 havinga thickness of 300 nanometers was deposited on the substrate 3 by asputtering process. The conditions of the sputtering process were asfollows;

    ______________________________________                                        Temperature of substrate                                                                            650° C.                                          Sputtering gas                                                                Ar                    8 sccm                                                  O.sub.2               4 sccm                                                  Pressure              5 × 10.sup.-2 Torr                                ______________________________________                                    

The Y₁ Ba₂ Cu₃ O_(7-x) oxide superconductor thin film 1 had twosuperconducting portions 11 and 12 on the horizontal planes 31 and 32 ofthe substrate 3, which were constituted of c-axis orientated Y₁ Ba₂ Cu₃O_(7-x) oxide superconductor thin films having c-axes of crystalsperpendicular to the horizontal planes 31 and 32, and one junctionportion 13 on the slope 33, which was constituted of a Y₁ Ba₂ Cu₃O_(7-x) oxide superconductor thin film having c-axes of its crystalperpendicular to the slope 33. Grain boundaries 41 and 42 were createdat the interface between the portions 11 and 13 and between the portions12 and 13.

Then, the Y₁ Ba₂ Cu₃ O_(7-x) oxide superconductor thin film 1 was etchedso as to form the pentagonal superconducting electrodes of thesuperconducting portion 11 and 12, and the narrow bridge having a length10 μm and a width of 5 μm of junction portion 13. Metal electrodes mightbe formed on the portions 1 1 and 12, if necessary. With this, theJosephson junction device in accordance with the present invention wascompleted.

A current-voltage characteristics of the above mentioned Josephsonjunction device was measured at a temperature of 85K. When a microwaveof 15 GHz and of 0.2 mW was irradiated, clear Shapiro steps of wasobserved at multiple voltages of 31 μV, and therefore, it could beascertained that the Josephson junction was realized in the device.

As explained above, the Josephson junction device in accordance with thepresent invention includes two superconducting electrodes ofsingle-crystalline oxide superconductor on horizontal planes of asubstrate, a junction portions of oxide superconductor on a slope of thesubstrate, which has a different crystal orientation, and a weak link ofgrain boundaries. The weak link of the Josephson junction device isformed of self-generated grain boundaries and there is no processing toform the weak link. Accordingly, the limitation in the fine processingtechnique required for manufacturing the Josephson junction device isrelaxed.

Embodiment 2

Referring to FIG. 3, there is shown a diagrammatic plane view of anotherembodiment of the Josephson junction device in accordance with thepresent invention. The Josephson junction device shown in thisembodiment is a dc SQUID, which has the same sectional structure as thatof the Josephson junction device of Embodiment 1. As shown in FIG. 3,the de SQUID includes an MgO (100) substrate 3 which is similar to thatof Embodiment 1 and a Y₁ Ba₂ Cu₃ O_(7-x) oxide superconductor thin film1 which forms a rectangular ring shape including two superconductingportions 11 and 12 and two junction portions 13 and 131.

The superconducting portions 11 and 12 of the Y₁ Ba₂ Cu₃ O_(7-x) oxidesuperconductor thin film 1 are positioned on the horizontal planes 31and 32 of the substrate 3 and formed of c-axis orientated oxidesuperconductor thin films. The junction portions 13 and 131 arepositioned on the slope 33 and formed of Y₁ Ba₂ Cu₃ O_(7-x) oxidesuperconductor thin films of which c-axes are perpendicular to the slope33.

Grain boundaries 41 and 42 are formed on the interfaces between theportions 11 and 13 and between the portions 12 and 13, and grainboundaries 411 and 421 are formed on the interfaces between the portions11 and 131 and between the portions 12 and 131. Each of the grainboundaries 41, 42, 411 and 421 forms a weak link of the Josephsonjunction, however, the neighboring grain boundaries 41 and 42, and 411and 421 are formed in close proximity to each other so that each of theneighboring weak links of the Josephson junction operates as one weaklink of the Josephson junction. These two Josephson junctions has almostthe same dimension so that their characteristics are homogeneous.

In order to manufacture the above dc SQUID, a rectangular ring shaped Y₁Ba₂ Cu₃ O_(7-x) oxide superconductor thin film 1 having a thickness of300 nanometers is deposited on the MgO (100) substrate 3 which has aslope 33 on its surface by using a patterned silicone plate as a mask.

As explained above, the dc SQUID in accordance with the presentinvention has two weak links of the Josephson junction which areconstituted of the grain boundaries. Therefore, it become easy to formweak links of the Josephson junction of which properties are linear up.Accordingly, it is easy to manufacture the dc SQUID with goodrepeatability, and the manufactured dc SQUID has a stable performance.

In the above mentioned embodiments, the oxide superconductor thin filmcan be formed of not only the Y-Ba-Cu-O compound oxide superconductormaterial, but also a high-T_(c) (high critical temperature) oxidesuperconductor material, particularly a high-T_(c) copper-oxide typecompound oxide superconductor material, for example a Bi-Sr-Ca-Cu-Ocompound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compoundoxide superconductor material.

The invention has thus been shown and described with reference to thespecific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the illustratedstructures but converts and modifications may be made within the scopeof the appended claims.

We claim:
 1. A Josephson junction device comprising a MgO singlecrystalline substrate including a principal surface having twohorizontal planes and a slope inclined at an angle of 5° to 30° betweenthe two horizontal planes, and an oxide superconductor thin film formedon the principal surface of the substrate, which includes a first and asecond superconducting portions of a first single crystalline oxidesuperconductor and a second single crystalline oxide superconductorrespectively positioned on the two horizontal planes of the substrate, ajunction portion of a single crystalline oxide superconductor having adifferent crystal orientation from the first and second superconductingportions positioned on the slope of the substrate and two grainboundaries between each of the first and the second superconductingportions and the junction portion, which constitutes the weak link ofthe Josephson.
 2. A Josephson junction device as claimed in claim 1,wherein the oxide superconductor thin film includes two junctionportions, each of which includes the grain boundaries which constitutethe weak link of the Josephson junction.
 3. A Josephson junction deviceas claimed in claim 2, wherein the oxide superconductor thin film isformed in a rectangular ring shape which includes the two junctionportions each of which includes the grain boundaries which constitutethe weak link of Josephson junction, so that the Josephson junctiondevice constitutes a dc SQUID.
 4. A Josephson junction device as claimedin claim 1, wherein the difference of heights of the horizontal planesof the principal surface of the substrate is 100 to 500 nanometers.
 5. AJosephson junction device as claimed in claim 1, wherein the oxidesuperconductor thin film is formed of c-axis oriented oxidesuperconductor thin film.
 6. A Josephson junction device as claimed inclaim 1, wherein the junction portion of the oxide superconductor thinfilm has a narrower width than the first and second superconductingportions.
 7. A. Josephson junction device claimed in claim 6 wherein thenarrow junction portion of the oxide superconductor thin film has awidth of 1.5 to 15 μm and a length of 5 to 50 μm.
 8. A Josephsonjunction device claimed in claim 1 wherein the oxide superconductor thinfilm is formed of a high-T_(c) (high critical temperature) oxidesuperconductor, particularly, formed of a high-T_(c) copper-oxide typecompound oxide superconductor.
 9. A Josephson junction device claimed inclaim 8 wherein the oxide superconductor thin film is formed of an oxidesuperconductor material selected from the group consisting of aY-Ba-Cu-O type compound oxide superconductor material, a Bi-Sr-Ca-Cu-Otype compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O typecompound oxide superconductor material.